This invention relates to an improved gas turbine power plant cycle and more particularly to a cycle wherein both the compressed motive fluid and water/steam are simultaneously and in-parallel heated by the exhaust gases between the turbine exit and stack.
In current practice, exhaust heat from a gas turbine power plant is recaptured for use in the heat-engine cycle by a single-mode device, i.e., one that heats compressor discharge air only such as a recuperator or regenerator; or one that heats water/steam only such as a waste heat boiler, with the steam utilized in the heat-engine either in a separate steam turbine (combined cycle) or by direct injection into the gas turbine combustor (Cheng cycle). The Cheng cycle is disclosed in U.S. Pat. No. 4,128,994. The present invention secures significant thermodynamic advantages by simultaneously recovering exhaust heat by both of the above modes in parallel and across a common exhaust gas temperature span. These thermodynamic advantages cannot be realized by the use of a boiler utilizing exhaust gases exiting from a recuperator, i.e., in a series arrangement, as currently practiced in some cogeneration applications. The advantages also cannot be fully realized solely by evaporative aftercooling followed by recuperation (e.g., Lysholm, U.S. Pat. No. 2,115,338) or by using staged evaporative water injection along recuperators without aftercooling (e.g., Foote, U.S. Pat. No. 2,869,324). Besides the superior thermodynamic performance over the latter two systems, the use of a separate boiler in the present invention also permits flexible integration into a cogeneration system.
It is therefore an object of the present invention to provide a novel configuration for the turbine exhaust heat recovery system in a gas turbine power plant cycle that provides high thermal efficiency.
Yet another object of the invention is a gas turbine power plant cycle resulting in flexible operation in a cogeneration plant having additional degrees of freedom for variable heat and power loads in cogeneration applications.